A closure of this type is known, for example, from U.S. Pat. No. 3,001,657. According to that patent, the cap--initially unthreaded--is slipped around the stepped and threaded bottle neck and is then deformed into mating engagement with the threads. Alternatively, in line with another conventional technique, one part of the cap--generally the upper one--may be prethreaded and screwed onto the corresponding neck portion whereupon the other part is deformed, e.g. by shrinkage, to interfit with its associated neck portion, thereby completing the seal.